Birds

In March 1956 somebody living in Buckinghamshire, in England, noticed two wrens go into a nesting-box one evening. The box was under the eaves of a wood-shed, in a well-sheltered position, visible from the window of the house. The next evening five wrens were seen to go in, and three evenings later fourteen were seen to go in to roost for the night.

The man who saw this wrote to a newspaper about it and two weeks later a reader replied saying that he had put wire-netting over his thatched roof to keep out the sparrows. The wrens, however, being so small, could squeeze under the netting, and night after night he watched as 52 wrens went in to roost in the same hole in the straw.

During the winter of 1962-3 Britain endured particularly severe weather. Wrens suffered badly and the following year over wide areas of the country, where the song of the wren is usually a familiar sound in summer, the air was silent. The wrens had been wiped out. Yet this species of wren has been known to nest in mountains and to pass the winter at heights of 4,000 ft. above sea-level. A wren has even been seen at 7,700 ft.

Perhaps the scientific name gives us a clue to this contradictory situation. A troglodyte is a cave-dweller, or somebody dwelling in seclusion. We are told the wren was called troglodytes because it makes a domed nest, like a hermit's hut. More likely it was because of its secluded habits. It may even be that the wren is more of a cave-dweller than we realize. Wrens can often be seen in caves or in cave-like recesses in quarries. There, they hang upside-down from the roof, searching for insects, spiders and millipedes. It may be that where caves are plentiful, as on a mountain, wrens can survive cold weather better than on lowlands with the bushes covered with snow. In any event, one thing that helps them is their habit of clumping in a roost, and in the cold winter of 1962-3 there was a nesting-box into which about fifty wrens were seen to fly one evening. When the nesting-box was opened carefully, so as not to disturb the birds, it was seen that they were in three tiers, each in a circle, one on top of the other. In each circle the bird faced inwards, so that, in addition to conserving heat by crowding together, the warmth from their combined breath must have produced a warm 'core' in the heap made by their bodies.

POOR-WILL STARTLES THE WORLD

There have been many reports during past centuries of birds found hibernating. In the 18th century Gilbert White, the celebrated English naturalist, had written two stories about swifts and swallows found asleep, the swifts in a church tower, the swallows in a cliff, in winter. There had been other reports, too. But all were treated as impossible, and the very idea of birds hibernating was treated as ridiculous until 1946.

In that year, on December 29, in the Chucka-wella Mountains of the Colorado Desert, Professor Edmund C. Jaeger, of the Riverside College, California, made a startling discovery. With two of his students Professor Jaeger saw a poor-will, a species of nightjar, resting head upwards in a hollow in the face of a wall of rock some 2% feet up from the sandy bottom of a slotlike canyon. The poor-will's plumage blended with the mottled surface of the granite, so that it could easily have escaped attention. It was only a slight hollow, so that the back of the bird was flush with the surface of the rock. The professor and his students could hardly believe their eyes, for this was the season when all good poor-wills should have gone to southerly latitudes for the winter. The party watched the bird for ten minutes but it gave no sign of life. Then they stroked its feathers. Still there was no movement and they began to speculate whether it was dead or sick - or in winter sleep. They went away. Two hours later they returned. The bird was still there in the same position. Professor Jaeger picked it up, turned it about in his hands. Still there was no sign of life. It felt light and the feet and eyelids were cold. The party shouted in the hope of rousing it, and receiving no response to this they placed it back in the hollow. The poor-will lazily opened one eye and shut it again.

Two days later Professor Jaeger returned, this time with Mr. Lloyd Mason Smith. The poor-will was still there in exactly the same position. As the professor picked up the bird it puffed a few times as if expelling air from its lungs, opened its eyes and began to squeak like a mouse. It yawned, started to whine, and raised its wings high over its back, holding them rigidly there. A fully active nightjar will bring its wings up into this position, momentarily, even when in flight, as part of its aggressive display. Even when in a deep sleep, presumably, the reflex still can function. The two men tried to push the wings back into place, but as fast as they folded them down they sprang up again. At last they succeeded in pushing them into place, replaced the bird, somewhat ruffled, and departed. In the afternoon they came back. The poor-will was still there, its feathers ruffled. Mr. Smith picked it up to photograph it. The bird flew away.

A year later, on 26 November, 1947, Professor Jaeger returned to the same spot. A poor-will was there again. It was probably the same bird. On December 8 he returned and this time put a ring on its leg so that he should know it for the same bird if he found it there next year.

The following winter Professor Jaeger returned. The bird was there again. This time the professor was prepared to push his investigations further. He took the bird's temperature a number of times on succeeding days. He found this was more or less constant, well below normal, and was lower than that of the surrounding air, which varied from day to day. He listened for its heart-beat with a stethoscope and heard nothing. He had held a mirror before the beak and detected no sign of breathing. There were no visual signs of breathing either. In fact, the bird showed all the characteristics of true hibernation. The matter was no longer in doubt. At least one poor-will had been proved beyond a peradventure to be capable of hibernation.

HUMMINGBIRDS' ALTERNATIVE TO SLEEP

John Gould, the celebrated 19th century ornithologist, was the first to notice that hummingbirds spent the night in a state of torpor. He reported that he found the birds sitting on their perches, their heads drawn down into their shoulders, their beaks pointing forward, not turned back and tucked into the feathers, to all appearances lifeless. Moreover, he was able to move them about without their waking, and when laid on a table they looked like so many dried skins. Yet the air temperature was between 63° and 70° F, which is for us a comfortable room temperature.

Later researches proved that this torpidity had nothing to do with cold. It is the hummingbirds' substitute for sleep. When any animal sleeps it uses less energy than when awake and moving about. Strange as it may sound, an animal that is torpid uses even less energy, and moreover less energy is required for it to become fully awake. In a state of torpor the body is sluggish and inert but the senses remain to a large extent still awake. The reason why a bird broods its eggs is to keep them from getting cold. A torpid bird sitting on her eggs would be useless. So when a hummingbird is incubating she does not become torpid.

SWIFTS AND SWALLOWS IN A DAZE

From the earliest times in Europe there have been beliefs that some birds hibernated. As the science of ornithology became established on a firm basis, in the 19th century, it was generally assumed that these stories sprang from an attempt to explain why birds seen in summer disappeared in winter. In other words, the modern ornithologist saw in this an ignorance of the phenomenon of migration. This may be correct, yet the fact remains that for about a century, until the mid-twentieth century, ornithologists were emphatic that no birds hibernated. We now know this is equally incorrect. It is interesting therefore to review some of the accounts of swifts and swallows found in a state of torpor.

Gilbert White, writing in 1767, told of a clergyman friend who, as a grown boy, was present when workmen were pulling down the battlements of a church tower early in the spring. Among the rubble were found two or three swifts which seemed at first appearance to be dead, but on being carried towards a fire they revived. Unfortunately, the boy was so determined to give the birds the best attention, by keeping them in a paper bag hung close to the fire, that they were suffocated.

Two years after this, Carew, writing about Cornwall, said that in the west parts of that county, during the winter, swallows are found sitting in old deep tinworks and in holes in the sea cliffs.

It may be that Carew was talking about swifts because even today some countrymen in England still speak of swifts as swallows.

But whatever he meant neither he nor White found any substantial support for their stories among men of science who came, later.

In 18th century England there was also a belief that in winter swallows became torpid and conglobulated into a mass in the mud at the bottoms of ponds. This was before it became known that swallows and swifts spent half the year in Africa, and that when they appear in England in spring it was because they had migrated thousands of miles north to breed, and that in autumn they disappeared south again.

Well over a century later Sir John McNeill and Sir Henry Rawlinson reported having seen hundreds of swallows dormant in burrows in Persia. Such was the prejudice against any idea of birds hibernating, or even appearing to be hibernating, that nobody believed them.

The American ornithologist Wilson C. Hanna, fared better. He reported that in the severe cold weather of winter, 1913, in the cliffs of Slover Mountains, in California, he found eight white-throated swifts in a torpid state. He had taken them out of a crevice where, with others, they were roosting, as Hanna put it, "in a dazed or numb state." Hanna took them into a warm room where they became active, and flew away when they were later released.

TEMPORARY HIBERNANTS

Swifts spend part of the year in Africa, coming to Europe in spring to breed and returning south in summer. They nest in towers and roof-spaces, and when the nestlings are hatched the parents spend the whole day on the wing catching flying insects to feed themselves and their brood. When there are spells of heavy rain flying insects are beaten down and the food-supply of the swifts is prejudiced.

It is noticeable that when heavy black clouds appear in the distance swifts can be seen flying away in a stream in the opposite direction to which the storm clouds are approaching. This had been more especially noted in Finland, where swifts are known as storm-swallows or rain-swallows. Elsewhere they have been called storm-birds, even devil-birds.

What happens is that the adult swifts may fly anything up to 800 miles out of the path of an advancing storm. In 1950, a Finnish scientist, J. Koskimies, investigated what happens to the nestling swifts while the parents are gone. He found they went into a temporary hibernation.

The temperature of the body of an adult swift is constant around 104° F. That of the nestling swift, by contrast, is normally lower than this and, moreover, fluctuates with the rise and fall of the temperature of the surrounding air. In periods of storms, when the air temperature drops to about 42°F, the nestlings sink into a semi-comatose condition. Their breathing and pulse-rate fall to a very low figure and they are in a condition closely similar to that of a normal hibernant. They can survive this condition of starvation and cold for as long as ten days during which they may lose halt their body-weight. The adult swift can endure starvation for only half this time and can lose only one-third of its body-weight before succumbing.

SLEEPING IN THE COLD

When we go to bed we cover ourselves with a pile of bedclothes, to keep warm while we sleep. When a bird sleeps it fluffs out its feathers so increasing the amount of air trapped among them. This gives greater insulation. But experiments have shown that fluffing out the feathers is not enough to keep birds warm on a cold night. During the day a bird, like any other warm-blooded animal, maintains a constant body-temperature by increasing the amount of heat in the body when the temperature of the surrounding air drops. It does this by increasing its muscular activity. That is, by taking exercise and generally moving about more. It also receives some heat direct from the sun, or from the air warmed by the sun.

At night the sun is gone and the bird is at rest, and therefore inactive, so the two main sources of heat are lost. What the bird can do, and does do, is to step up its temperature by shivering. The slight movement of the muscles used in shivering is a partial substitute for exercising the muscles by moving about. Under test birds have been seen to shiver for 20 seconds or so at intervals varying from 2 to 10 minutes and to keep this up for the two hours during which they were being watched. For these tests the birds were placed in glass cages small enough so that they could not fly but large enough so that they were not cramped. The cages were placed in darkened rooms, so the birds were at rest. The breathing, shivering and other activities were recorded on instruments.

It would be practically impossible to repeat the experiments with a bird actually asleep, or even to tell whether or not the birds fell asleep during such experiments. It is virtually impossible to watch a bird closely enough when it is sleeping because it so readily wakes up if a light is shone on it. All the same, it seems reasonable to suppose that the shivering would continue while the bird was asleep. Although a bird's feathers keep the body warm, nevertheless there is a loss of heat from the surface of the plumage. This loss is high in small birds and more or less negligible in large birds. This is why, as we saw in the article on the robin, small birds roost in a shelter or clump together. By contrast, larger birds may sleep exposed, often roosting on a branch in the open when snow is falling. Owls and pigeons will sleep with snow on their feathers.

In addition to loss of heat from the surface of the feathers, there are several spots on the bird's body which are warm spots, from which heat is more readily lost than from the rest of the body. One of these is on the angle of the wing. Another is around the eye, and a third is around the beak. It has been suggested that the warm spot around the eye is especially necessary for birds living in cold regions, because a bird flying into a cold wind needs some device for keeping the water in the eye from freezing. In the same way, it is suggested, the warmth round the beak is needed, especially in cold regions, to warm air that is being inhaled. Whether these things are correct or not, the fact that there are these warm spots is a good reason why small birds should bury their heads in their feathers at night. The larger the bird the less need for such precautions, since the heat-loss from these warm spots is relatively much less.